Organic electroluminescent (EL) devices, or organic light-emitting diodes (OLEDs), are electronic devices that emit light in response to an applied potential. The structure of an OLED includes, in sequence, an anode, an organic EL unit, and a cathode. The organic EL unit disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al., “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987), and commonly assigned U.S. Pat. No. 4,769,292 demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly includes a host material doped with a guest material wherein the layer structures are denoted as HTL/LEL/ETL. Further, there are other multilayer OLEDs that contain more functional layers in the devices. At the same time, many kinds of EL materials are also synthesized and used in OLEDs. These new structures and new materials have further resulted in improved device performance.
EL devices in recent years have expanded to include not only single color emitting devices, such as red, green and blue, but also devices that emit white light. Efficient white light producing OLED devices are highly desirable in the industry and are considered as a low cost alternative for several applications such as full color displays, paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
An OLED is actually a current driven device. Its luminance is proportional to current density, but its lifetime is inversely proportional to current density. In order to achieve high brightness, an OLED has to be operated at a relatively high current density, but this will result in a short lifetime. Thus, it is critical to improve the luminous efficiency of an OLED while operating at the lowest possible current density consistent with the intended luminance requirement to increase the operational lifetime.
In addition to the continued need to provide OLEDs having improved lifetime, it is desirable to have OLED devices with good operational performance in luminance and voltage stability over the lifetime of the OLED device under varying operating conditions. For practical applications, OLED devices should have high luminance stability and voltage stability under ambient conditions as well as higher operating temperature conditions (such as greater than 85° C.).
In many electronic systems, e.g., in some active matrix display designs, the available voltage is limited and the power consumption is directly proportional to the voltage required to drive the OLED and the TFTs. Thus, there is a need to reduce the voltage necessary to drive the OLED. One way to lower the OLED driving voltage is to provide an electron-injecting layer (EIL), which typically includes an electron-transporting material doped with an n-type dopant such as a low-work function metal. For example, see U.S. Pat. Nos. 6,013,384, 6,509,109, 6,566,807, and 6,589,673. However, the high temperature operational stability is a major issue. The lower softening temperature (Tg) of the organic materials limits the use of this device in high temperature conditions. Also if the device is designed for high temperature operation, it does not necessarily function well under ambient conditions.